Methods for eliminating magnetic field and temperature related errors in magnetic sensors used for the measurement of coating thickness

ABSTRACT

Methods for eliminating error in magnetic sensors used for measuring a coating thickness caused by static or changing external magnetic fields or temperature. The methods involve measuring an output voltage of a magnetic sensor, corresponding to an internal resistance of the magnetic sensor, in a static or changing magnetic field or external temperature, storing the value of the output voltage, performing mathematical operations with the stored value of the output voltage, and correcting the output voltage of the magnetic sensor to accurately indicate a coating thickness.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application which claims the benefit ofapplication Ser. No. 10/087,216, filed Mar. 4, 2002 now U.S. Pat No.6,724,187. The disclosure of the prior application is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a method to measure non-magnetic coatings onferro-magnetic substrates, using a magnetic sensor, where errors due tooffset voltage of the magnetic sensor, temperature dependence of thesensor parameters and disturbing magnetic fields are compensated.

DESCRIPTION OF THE PRIOR ART

Patent EP0028487 discloses a measuring device that uses a Hall-sensortogether with a permanent magnet. In this device the flux of externalmagnetic fields is superimposed to the flux of the internal permanentmagnet and can not be separated. Therefore the change of the outputsignal of the Hall-sensor due to these external fields can not bedetermined. This results in erroneous measurements.

Pat. DE19910411 discloses a method for offset compensation ofHall-sensors. This method eliminates the offset by applying twodifferent currents to the Hall-sensor, measuring the Hall-voltage ateach current and calculating the offset from these two measuredvoltages. However this method does not allow for compensation ofexternal magnetic fields.

In Pat. US33359495 a device is described that has a Hall-sensor with acoil arranged around the Hall-sensor. This device uses alternatingelectromagnetic fields generating eddy currents in the substrate. Theseeddy currents strongly depend on the conductivity of the substrate. Theeddy currents in turn generate a secondary magnetic field opposed andsuperimposed to the primary field. This results in a change of theoutput voltage of the Hall-sensor. Since the strength of the eddycurrents and consequently the strength of the secondary magnetic fieldstrongly depend on the conductivity, this has a negative influence onthe measuring results. An additional error occurs when the substrate iscovered with a conductive coating. Also in the conductive coating eddycurrents are generated. They have the same effect on the flux throughthe Hall-sensor as those generated in the substrate.

SUMMARY OF THE INVENTION

It is an object of the invention to provide simple methods which allowto eliminate the different error sources when using magnetic sensors forthe measurement of coating thickness.

With the first two methods the influence of external magnetic fields, asthey can be present in mechanically or thermally treated substrates, orof time varying fields on the measuring device can be eliminated.Additionally this invention makes use of the advantages of the staticmagnetic field being insensitive to the conductivity of substrate andmetal coatings.

A further advantage of this method is the automatic compensation of thetemperature dependent offset voltage of magnetic sensors. This offsetvoltage occurs when applying a current to the sensor, even in theabsence of a magnetic field.

A further described method of this invention can be used to compensatethe influence of temperature on the signal voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the measuring device.

FIG. 2 is a circuitry block diagram of the magnetic sensor forcompensation of the temperature dependence of the signal voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The measuring device is equipped with a magnetic sensor 10 and a coil 20disposed in the neighbourhood of the sensor. The control unit 30contains the sensor electronics 31 to feed the magnetic sensor 10 with acurrent I and to receive the signal voltage U. The control unit 30 alsocontains the coil control unit 32 to feed the current I_(c) through thecoil 20.

The compensation of external magnetic fields as well as the offsetvoltage of the magnetic sensor 10 can be performed by either of themethods described as preferred embodiments hereafter.

With the first method the output voltage U in the magnetic sensor 10 ismeasured first by the magnetic sensor control unit 31 without a currentI_(c) through the coil 20, when the measuring probe 15 is placed on theobject under test 50. The output voltage U depends on the externalmagnetic field B_(ext) through the magnetic sensor 10. This outputvoltage U₁, is transferred from the control unit 30 to the evaluationunit 40. There it is digitised and stored. With the next step a currentI_(c) is feed through the coil 20. This current generates a magneticfield B that is superimposed to the external magnetic field B_(ext).This creates a different magnetic flux through the magnetic sensor 10and consequently a different output voltage U₂. This voltage U₂ istransferred from the control unit 30 to the evaluation unit 40. There itis digitised and stored. A linear relation exists between output voltageU and magnetic flux B through the magnetic sensor which can be writtenin the following form:U=U ₀ +K·B·I  (1)

U₀ is the offset voltage, K is the sensor constant and I is the currentthrough the magnetic sensor 10. Equation 1 immediately shows that theoutput voltage is the sum of the offset voltage and the sum of allmagnetic flux components B_(a), B_(b), . . . through the magnetic sensor10:U=U ₀ +U _(a) +U _(b) + . . . =U ₀ +K·(B _(a) +B _(b)+ . . . )·I  (2)

Therefore in the evaluation unit 40 the following difference can becalculated:U _(res)=(U ₂ −U ₁)=[U _(o) +K·(B+B _(ext))·I]−[U _(o) +K·B _(ext)·I]=K·B·I  (3)

The result U_(res) represents the voltage the magnetic sensor wouldgenerate if no external magnetic field were present. Simultaneously theoffset voltage U₀ is eliminated. Using the functional relation betweenoutput voltage U of the magnetic sensor 10 and the distance of thesensor from the substrate (=coating thickness d=f(U)) the true coatingthickness d can be calculated from the result U_(res).

With the second method a certain current I_(C1) is feed through the coil20 by the coil control unit 32 when the measuring probe is placed on theobject under test. The magnetic field B₁ generated by this current,together with an external magnetic field B_(ext), generates a magneticflux through the magnetic sensor 10, resulting in an output voltage.U ₁ =U ₀ +K·(B ₁ +B _(ext))·I  (4)

This output voltage U₁ is transferred from the control unit 30 to theevaluation unit 40. There it is digitised and stored. With the next stepa second current I_(C2) is feed through the coil 20. In a preferredembodiment the current is selected as I_(C2)=−I_(C1). The absolute valueof the magnetic field B₂ is the same as that of B₁, but with reversepolarity: B₂=−B₁. The magnetic field B₂, together with the externalmagnetic field B_(ext) generates a magnetic flux through the magneticsensor 10, resulting in an output voltage:U ₂ =U ₀ +K·(B ₂ +B _(ext))·I=U ₀ +K·(−B ₁ +B _(ext))·I  (5)

This output voltage U₂ is transferred from the control unit 30 to theevaluation unit 40. There it is digitised and stored. Using equation 2and the two digitised output voltages U₁ and U₂, the voltage resultingfrom the external magnetic field B_(ext) and the offset voltage U₀ canbe eliminated as follows:U _(res) =[U ₁ −U ₂ ]=K·[(B ₁ +B _(ext))−(B ₂ +B _(ext))]·I=K·[B ₁ −B ₂]·I  (6)

Because of B₂=−B₁, =B equation 6 can be rewritten as:U _(res)=2·K·B ·I  (7)

Using the functional relation between output voltage U of the magneticsensor 10 and the distance of the sensor from the substrate (=coatingthickness d=f(U)) the true coating thickness d can be calculated fromthe result U_(res).

In a preferred embodiment the influence of time varying magnetic fields,which usually have typical frequencies such as 60 Hz (e.g. thosegenerated by transformers), can be eliminated by repeated changesbetween those two currents I_(C1) and I_(C2). The resulting outputvoltage is determined using the following equation:U _(res) ={[U ₁ −U ₂]₁ +[U ₁ −U ₂]₂ + . . . +[U ₁ −U ₂]_(N) }/N =K·{[(B₁ +B _(ext1))−(B ₂ +B _(ext2))]₁+[(B ₁ +B _(ext1))−(B ₂ +B _(ext2))]₂+ .. . +[(B ₁ +B _(ext1))−(B ₂ +B _(ext2))]N}/N·I =2 K·[B+{(B _(ext1) −B_(ext2))₁+(B _(ext1) −B _(ext2))₂+ . . . +(B _(ext1) −B _(ext2))_(1N)}/N]·I  (8)with N being the number of repetitions. Those components of externalmagnetic fields in curved brackets will be averaged to zero, especiallywhen the N pairs of measurements are taken in an interval that which isequal to one or several periods of the time varying magnetic field.

A further method in accordance with this invention allows to compensatethe temperature dependence of the output voltage U. The temperature inthe magnetic sensor 10 or the temperature change with respect to areference temperature can be determined by measuring the internalresistance 11 of the magnetic sensor 10. This can be used to determine acompensation factor to correct the output voltage for temperaturechanges.

Starting from equation 1 the temperature gradient of the output voltagecan be calculated as follows (a change in offset voltage can beneglected as effect of second order):dU/dT=B·[dK _(H) /dT·I+K·dI/dT]=B·K·I·[α+β]  (9)using the following abbreviations:dK/dT=K(T ₀)·α, dI/dT=I(T ₀)·βK(T₀) und I(T₀) are related to reference temperature T₀.

α and β are the temperature coefficients of the output voltage U and thesensor resistance 11 respectively. Since the coefficients α and β areknown for each individual type of magnetic sensor, these parameters canbe implemented directly in the control unit or used as parameters in adigital control unit.

The temperature dependant output voltage can be determined using thefollowing equation:U(T)=U(T ₀)+dU/dT·(T−T ₀)=B·K(T ₀)·I(T ₀)·{1+[α+β]·(T−T ₀)}  (10)

Equation 10 shows that the output voltage, measured at a temperature T,needs to be corrected by a factor {1+[α+β]·(T−T₀)} as given in equation10, in order to reduce the output voltage to the correct value atreference temperature T₀. To do so the evaluation unit 40 has tocalculate the factor in curved brackets which is transferred via thecontrol unit 30 to the control of the magnetic sensor 31 to adjust thecurrent I through the magnetic sensor by this factor.

To determine the temperature T the voltage drop across the sensorresistance 11 is measured by the sensor control unit 31 by feeding aconstant current through this resistance 11. First this is done attemperature T₀ as a reference, and then at each measurement of theoutput voltage U. To calculate the actual temperature T or thetemperature deviation ΔT from the reference temperature T₀ the followingequation is used:R(T)=R(T ₀)·[1+β·(T−T ₀)]=R(T ₀)·[1+β·ΔT]  (11)β is the temperature coefficient of the sensor resistance 11 givenabove.

Then the temperature difference relative to T₀ is calculated as:ΔT=(1/β)·R(T)/R(T ₀)  (12)

The procedure to determine a temperature compensated coating thicknessis as follows:

First with a reference measurement the sensor resistance R(T₀) isdetermined in the control unit 30. This value is digitised and store inthe evaluation unit 40. Each time a coating thickness measurement istaken the value R(T) is determined first, transferred from the controlunit 30 to the evaluation unit 40, where it is digitised and stored.This value is then used to calculate the temperature difference ΔTaccording to equation 12. Subsequently, using ΔT, the correction factor{1+[α+β]·(T−T₀)} is calculated in the evaluation unit 40 and transferredto the control unit 30. Then this parameter is used in the magneticsensor control unit 31 to adjust the current I such, that the outputvoltage conforms to the respective voltage at reference temperature T₀.

Alternatively to adjusting the current I through the resistor 11 of themagnetic sensor 10 upon temperature change the correction of the outputvoltage U at the measured temperature T can also be done digitally inthe evaluation unit 40 using equation 10 and 12.

Of course it is obvious for a person skilled in the art to combine themethod for temperature compensation of the output voltage with one ofthe methods for compensation of external magnetic fields.

1. A method to compensate for temperature dependence of a measuringdevice for measuring the thickness of a coating comprising the steps of:using a magnetic sensor element as the measuring device; receiving atemperature signal corresponding to the internal resistance of themagnetic sensor element; determining a correction factor using thetemperature signal and temperature coefficients of the magnetic sensorelement; and correcting an output signal of the magnetic sensor elementusing the correction factor.
 2. The method of claim 1, wherein theoutput signal of the magnetic sensor element is corrected by applyingthe correction factor to an input signal of the magnetic sensor element.3. The method of claim 1, wherein the correction factor is determined bycalculation, wherein the correction factor is determined by thefollowing relationship:1+[(α+β)×(T−T _(o))], and wherein α is a temperature coefficient of anoutput voltage of the magnetic sensor element, β is a temperaturecoefficient of the internal resistance of the magnetic sensor element. Tis an ambient temperature, and T_(o) is a reference temperature.
 4. Themethod of claim 1, wherein the magnetic sensor element is a Hall-sensorelement.
 5. The method of claim 1, wherein the magnetic sensor elementis a GMR-sensor element.
 6. The method of claim 1, wherein thetemperature signal corresponds solely to the internal resistance of themagnetic sensor element.
 7. The method of claim 6, wherein the outputsignal of the magnetic sensor element is corrected by applying thecorrection factor to an input signal of the magnetic sensor element. 8.The method of claim 6, wherein the correction factor is determined bycalculation, wherein the correction factor is determined by thefollowing relationship:1+[(α+β)×(T−T _(o))], and wherein α is a temperature coefficient of anoutput voltage of the magnetic sensor element, β is a temperaturecoefficient of the internal resistance of the magnetic sensor element, Tis an ambient temperature, and T_(o) is a reference temperature.
 9. Themethod of claim 6, wherein the magnetic sensor element is a Hall-sensorelement.
 10. The method of claim 6, wherein the magnetic sensor elementis a GMR-sensor element.